Polystyrene foam, particularly in sheet form, is presently being made from a number of blowing agents which have many undesirable characteristics. Volatility, flammability, poor thermoforming qualities, brittle physical properties, high cost, or an adverse affect to the ozone layer are just a few. Examples of the blowing agents that produce these characteristics in the production of polystyrene foam would include aliphatic hydrocarbons and fully or partially halogenated hydrocarbon.
A variety of normally gaseous or liquid blowing agents have been proposed for olefinic or styrenic polymers, including virtually all of the common atmospheric gases and lower hydrocarbons.
For polystyrene, the C.sub.4 -C.sub.6 alkanes have gathered widespread acceptance, especially pentane. Following a typical extrusion foaming step, the stock material is ordinarily aged before thermoforming into containers or the like. During aging, the foam cells and polymeric matrix become partially depleted of volatile hydrocarbons, which enter the atmosphere. However, potential atmospheric contamination by these by-products of foam manufacture has led workers to seek non-polluting alternative blowing agents, such as the usual atmospheric gases, e.g., nitrogen, carbon dioxide.
U.S. Pat. Nos. 4,344,710 and 4,424,287 disclose blowing agents which are blends of carbon dioxide and aliphatic or fully or partially halogenated hydrocarbons. These patents, however, state that the use of 100% carbon dioxide as a blowing agent has not been successfully employed due to the extreme volatility. Use of these materials is said to produce corrugation and surface defects.
Others have developed methods and apparatuses for injecting a gaseous blowing agent into a molten resin charge, such as polystyrene in an accurate and precise manner. One example of this can be seen in U.S. Pat. No. 4,470,938.
Numerous experiments have been aimed at first preparing a masterbatch mix of a plasticized blowing agent and nucleator in which the blowing agent is uniformly distributed, and subsequently this premixture is added to polystyrene which is then extruded into foam. However, it has been shown that it is not possible to mix polystyrene with such a blowing agent without premixture foaming, since the blowing agent decomposes at the melt temperature of the polystyrene. This is an undesirable effect.
U.S. Pat. No. 4,940,735 teaches preparation of a masterbatch containing 30 to 80 weight percent of a plasticizer, 20 to 70 weight percent of blowing agent, and 10 to 20 weight percent of a cell regulator. While this masterbatch mix is an improvement over earlier art, it does not have all of the advantages of the present invention.
Thus, the object of the present invention was to develop a masterbatch mix for the preparation of polystyrene foams which when used platicizes, improves impact, improves melt flow and strength, and nucleates the inert gases in polystyrene in order to shorten extrusion process time. Also, the masterbatch mix must be utilized with CO.sub.2 or Nitrogen as the blowing agent. Moreover, the polystyrene foam produced have very fine cells and low densities, thus, thermoforming is easier.
The present invention relates to a process for producing polystyrene foam, said foam being prepared by extruding a styrenic polymer and a predetermined amount of masterbatch mix in conjunction with carbon dioxide, nitrogen or mixtures thereof as a blowing agent. The masterbatch plasticizes, improves flow rate, melt strength and nucleates the inert gases in the polystyrene foam which in turn shortens process time. The resulting polystyrene foam has a substantially uniform cell wall thickness and improved polymer melt strength and improved density.